So, despite all the good advice, I recommend something different. I think you should sit your parents down, thank them for helping you by letting you live there for free for 4 years in college and tell them that you would like to pay them back for the rent, but after that you need to save for a condo, married life, etc. then you should figure out a decent amount probably $350 a month and pay them for 48 months of rent. If you give them $700 a month, you will be done in 2 years total, you have told them in advance what you are doing, you have paid off a debt that was obviously unnamed, but was nevertheless a debt. You get to be honorable, they helped you graduate debt free.
After you have told them this, you can mention that it seems odd that you are giving money to them and they are giving money to your sister, but it is their money and they can do whatever they want with it. It is a lot of freaking money but you are paying off a debt. Tell them along the way, how much you have given and how many months you have left. Encourage them to save it for themselves. Make it clear that it ends 24 months after you started in January. You need to start your life, they should see that. So pay them back.
I know this may not be popular advice, but these are his parents, and obviously the "you can live here for free in college" was a lie. He is making decent money, he can afford to pay his parents back. Everyone wins. He tells them what to expect from him, they get some time to make their own decisions. He can even tell his siblings that this is what he is doing. It sucks that parents don't treat all their children the same, but life isn't fair, so pay your debt and be done.